Digital multimedia data such as high definition (HD) video and music can be transmitted to televisions for display. The multimedia can be formatted in accordance with Moving Pictures Expert Group (MPEG) standards such as MPEG-1, MPEG-2 (also used for DVD format), MPEG-4 and other block based transform codecs. Essentially, for individual video frames called “intraframes” or “I-frames”, these multimedia standards use Joint Photographic Experts Group (JPEG) compression. In JPEG, the image of a single frame is typically divided into small blocks of pixels (usually 8×8 and/or 1 6×16 pixel blocks) that are encoded using a discrete cosine transform (DCT) function to transform the spatial intensity values represented by the pixels to spatial frequency values, roughly arranged, in a block, from lowest frequency to highest. Then, the DCT values are quantized and compressed using run-length coding (essentially, storing a count of the number of, e.g., zero values that appear consecutively, instead of storing all the zero values).
Motion pictures add a temporal dimension to the spatial dimension of single pictures. MPEG is essentially a compression technique that uses motion estimation to further compress a video stream.
With more specificity, MPEG compression in general uses three kinds of video frames. Naturally, the above-mentioned I frames, in which the entire frame is a JPEG image composed of compressed, quantized DCT values, must be provided. But in MPEG compression the remaining frames (e.g., 59) that make up the rest of the video for that second are very much smaller frames that refer to the intraframes, in accordance with MPEG compression principles. In MPEG parlance these frames are called “predicted” frames (“P frames”) and “bidirectional” frames (“B frames”).
Of relevance to the present invention is that predicted frames contain motion vector references to the preceding intraframe or to a preceding predicted frame or B frame. If a block has changed slightly in intensity or color, then the difference between the two frames is also encoded in a predicted frame. Accordingly, MPEG computes motion vectors as part of rendering a stream, and these motion vectors, as recognized herein, are obtainable from the streams.
With respect to the above-mentioned HD video, a display having a 4:3 aspect ratio (width to height of display) such as a standard definition (SD) display that seeks to play HD video must essentially convert the 1,080 or 720 lines per image used in HD video to 480 lines per image used in SD displays or HD 4:3 displays. When the HD video has an aspect ratio of 16:9, a common feature of HD multimedia, the SD display must further trim the side portions of each image. Accordingly, so-called “pan and scan” has been introduced to selectively focus the picture received by a standard receiver on different portions (horizontal) of the HDTV picture, i.e., to pan and scan. Without a pan and scan feature, a 4:3 display will always display only the same fixed portion of HDTV picture (usually the center portion) to the exclusion of information at the edges of the HDTV picture.
In U.S. Pat. No. 4,943,856, incorporated herein by reference, a pan-scan feature is proposed in which data words are placed in the vertical blanking period of each frame of the HDTV signal. The data words are error-coded representations of the proportional relationship between the left and right panels of the HD video picture, and are used to scan the display either left or right over the picture.
As recognized by the present invention, not all HDTV programs are accompanied by the VBI pan-scan information. And, not all standard TVs are programmed to use the data words in the VBI, even if they are present. Nonetheless, the present invention recognizes a need for providing pan-scan of an HD image with a 16:9 aspect ratio in a 4:3 display, particularly under certain circumstances, so that interesting video information that otherwise would not be displayed is, in fact, presented to the user.